The Da Vinci glow

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When you think of Leonardo Da Vinci, what's the first thing that comes to mind? It probably isn't the science of climate change – but Da Vinci's insight into an ancient mystery has provided today's scientists with a simple yet effective tool for understanding climate change and its effects.

The old moon in the new moon's arms

When you look up at a crescent moon, you'll see the crescent in white, lit up by the sun. You'll also see the rest of the Moon, less bright but still visible. The cause of this phenomenon puzzled astronomers for years, and went by many names. It has been called 'ashen glow' and more poetically 'the old moon in the new moon's arms'. After Da Vinci's discovery that it's the Earth lighting up the dark side of the Moon, this light was named 'the Da Vinci glow'. It is sometimes referred to as 'earthshine', and when it occurs on other planets' moons: 'planetshine'.

What causes earthshine?

Earthshine is when the sun's light is reflected by the Earth's surface and illuminates the dark side of the Moon. Since the light has been reflected, it is much dimmer than the sunlight visible in the crescent moon. The brightness of the light is also affected by the Moon's 'albedo'. This is a measurement of a celestial object's reflectiveness, and ranges from 0 to 1. The Moon's average albedo is 0.12, and the Earth's is 0.3 – meaning that the Moon reflects about 12% of the sunlight that hits it, while the Earth reflects 30%. This means that the Earth seen from the Moon would look about 100 times brighter than the Moon seen from the Earth.

The Earth's reflectivity

Da Vinci wrote in his Codex Leicester (circa 1510) that he believed the Moon's "ghostly glow" to be a result of the sun's light reflecting off the Earth's oceans, and that the Moon is so reflective because it is itself covered with water. Although he was right about earthshine, he wasn't quite right on the detail. It is in fact clouds, not oceans, which make the Earth so reflective – that's why when Earth is seen from space, the clouds are white and the oceans are dark. Scientists are also fairly certain that there is no water on the Moon; its 'seas' are made of ancient hardened lava. However, since Da Vinci was writing at a time when humanity had yet to discover parts of the Earth, let alone the Moon, we can cut him some slack.

Although he was right about earthshine, he wasn't quite right on the detail.

Earthshine and climate change

One of the most important implications of earthshine is in the observation and study of climate change. A pioneer in this field is Dr Peter Thejll of the Danish Meteorological Institute. Having previously helped to provide conclusive evidence of the greenhouse effect, Thejll is now using the concept of earthshine to monitor one of the greatest threats to our planet.

Dr Thejll is currently working on a global automatic system to measure and observe changes in the Earth's reflectivity, by observing Earthshine. Because the main factor in the Earth's reflectivity is cloud, and cloud is a climate indicator, the hope is that scientists will be able to record climate change over time, simply by observing the moon.

the hope is that scientists will be able to record climate change over time, simply by observing the moon.

This is an incredibly cost-effective option compared to satellites, which cost millions to design, build and launch. Satellites also deteriorate over time, and can occasionally go off course or malfunction.

By drawing on the simple wisdom of the past, climate scientists have found a way to work towards protecting our future. When Leonardo Da Vinci wrote his Codex Leicester and drew his sketch of the crescent moon, could he have imagined that, 500 years later, we would be using his beautifully simple theory to address one of the greatest challenges humanity has ever faced?

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